Pulse electronic switching system



s. VAN MIERLO Erm. 2,638,505

PULSE ELECTRONIC SWITCHING SYSTEM May l2, 1953 Filed Aaron 24, 1948 5Sheets-Sheet l Attorney May 12, 1953 Sf VAN MIERLO ETAL K PULSEELECTRONIC SWITCHING SYSTEM 5 Sheets-Sheet 2 Filed March 24, 1948 M1-y12, A1953 s. VAN MIERLo Erm. 2,638,505

PULSE ELECTRONIC SWITCHING SYSTEM Filed March 24, 1948 5 Sheets-Sheet 5usy 70M:- Gavfearae V k.- Inventors A ttor/1e y May 12, 1953 s. VANMIERLO ErAL 2,638,505

PULSE ELECTRONIC swI'rcHING SYSTEM Filed March 24, 1948 5 Sheets-Sheet 4LIQ/ooo F/G- 6 ILOO L/O v 'B2 v2 K4 v4 c/oV5 72?. 242- C8 l nventons5mm/sms MNM/60 www;

A ttbm e y May l2, 1953 s. vAN MlERLo ETAL 2,638,505

PULsEfELEcTRoNIc swITcHING SYSTEM Filed March 24, 194e s sheetsfsheet s,By v

v Harney Patented May 12, 1953 UNrrsoy STATES PATENT OFFICE PULSEELECTRONIC SWITCHING SYSTEM Stanislas Van Mierlo, Antwerp, Belgium, andBoris Derjavitch, Paris, France, assgnors to International StandardElectric Corporation, New York, N. Y., a corporation of DelawareApplication March 24, 1948, Serial No. 16,778 In France March 26,1947

Distributors have been used in the Baudet telegraph system in which twotelegraph substations are connected at a regular time interval by meansof a transmission system common to several sub-stations in the samegroup. Each pair of sub-stations has at its disposal this transmissionsystem at regulartime intervals for a short time interval during which asignal may be exchanged between the two stations.

In the case of the Baudot telegraph system the connection of thesub-station is made by means of mechanical distributors provided at bothends of the circuit whose brushes revolve synchronously. Althoughmechanical arrangements may be used in the present invention electronicmeans are preferred and according to a feature of the invention thedistributor consists of an electron beam striking successively a seriesof electrodes.

In the embodiment of the present invention de scribed the transmissionof voice currents -or other signals is made by means of impulses ofextremely short duration.

lIwo different ways of sending messages by means of impulses may beused: the impulses may be transmitted at regular time interval-s andmodulated in amplitude or they may be of 'constant amplitude but timemodulated.

It is known that this second type of pulses offers certain advantagessignal/noise ratio, easier ampliiication and adiustrnent of the outputlevel, but the first type leads to simpler embodiments and givessatisfactoryresults in most of the cases which will be considered inrelation to electronic switching, and this type of modulation will beused in the embodiment described.

Itis known that it is possible to obtain a sufficient approximation withtwo or three impulses for the highest frequency to be transmitted. Forthe transmission of speech 8,000 impulses per second are considered assuilicient. To obtain the :final message currents it is suicient toapply these impulses to a low pass lter which reconstitutes the originalvoice frequency.

It may be noted that in the case of telephony the subscriber set may besomewhat different from the sets used at present, the dial may forinstance be replaced by push buttons or other means by which thesubscriber may dial the number of the wanted substation.

One object of the present invention is a switching system for arelatively large number of subscribers in which means are yused to delaythe such as a better' speech impulses with a high degree of precision.

' According to one feature of the invention, there is provided for theseizure of the connecting circuits a circuit such that when thesubscriber lifts up his receiver the first impulse is directed to a freeconnecting circuit and the succeeding impulses oi the same subscribercan reach only the same connecting circuit, and such that when aconnecting circuit is busy the impulses from another subscriber aredirected to another connecting circuit, thus avoiding doubleconnections.

According to other rfeatures ci the invention means are provided todelay the impulses by successive selections according to the number 4ofthe wanted subscriber, a rst selection corresponding for instance to thehundreds, a second to the tens and a third to the units.

The circuits will also be described for supplying the various tones suchas busy'tone, ringing and ringing back tone etc.

Other features and objects of the invention will be apparent from thefollowing description of an embodiment described in relation with thedrawings in which:

Fig. 1 shows an electronic distributor.

Fig. 2 shows an embodiment comprising two synchronized electronicdistributors.

Fig. 3 shows a circuitin which a single electronic distributor is used.

Fig. 4 is the schematic of a circuit incorporating features 'of theinvention.

Fig. 5 shows ycircuits common to several subscribers.

Fig. 6 shows an embodiment of a connecting circuit.

Fig. 7 shows a register circuit and Fig. 8 shows how Figs. 5, 6 and 'Iare assembled together.

As explained above electronic distributors are used to connect togethersubscribersets belonging to a group of subscribers connected to the samedistributor. The distributor located in the ex change ofce is shownschematically in` Fig. l. On this figure M is the modulating electrodeof an electron gun, K is the cathode of the tube, F the electron beamwhich periodically sweeps a series of electrodes e respectivelyconnected to telephone lines by means of deflecting plates Pl and P2.

The electron beam i-s therefore successively connected to the differentlines and an impulse is applied to each linewhen the correspondingelectrode is struck by the beam. The intensity of the beam may be modiedby any known means and the impulses applied to the lines may also bevariable in amplitude so as vto transmit the message from the exchangeto the telephone sets successively connected to each line I.

When speech is to be sent in the other direction use is made of a commonelectrode E constitutedfor instance by a thin conductive coating or by agrid or by a perforated plate. electrode maybe traversed by the electronbeam if an appropriate potential is applied to it.

The surface of the electrodes e is such that they release secondaryelectrons when they. are

struck by the electron beam. Secondary electrons are collected by thecommon V.electrode E and cause potential fluctuations at a Common pointC connected to the ground through a resistance R.

The amount of secondary electrons collected by electrode E depends onthe difference -of potential between electrodes e and the commonelectrode E, that is to say on the potential of electrodes e with regardto the ground.

.In vthis way the potential variations of electrodes e produced by themicrophone of the subscriber set correspond to potential fluctuations atthe common point C and the Vmessage from the telephone set may be sentto the exchange by means of a series of amplitude modulated impulses.

Fig. 2 shows a schematic of a complete installation. The lines areconnected to the electrodes c of electronic distributors DI and D2 andthese electrodes are swept in synchronism by the electron beam fromcathodes K and K by means of deflecting plates PI, P2 and Pi, P'Zrespectively.

On Fig. 2 the electron beams are shown at the moment when they strikeelectrodes e4. If it is assumed that each electron beam rotates at aspeed of P revolutions per second, corresponding vto a period ofduration T, each telephone line i is characterized by the position oiits electrode e in the field of the beam and therefore by the instant tof period T when the beam strikes the electrode.

At the moment when the `beam rdistributor DI passes over an electrodesuch as em a potential will appear at the common point the value ofwhich depends on the potential of electrode cm i. e. on the voicecurrent of the microphone Pm of the subscriber.

It will be assumed that a subscriber Pm (not shown) connected to 'lineY'lm and .characterized by an instant tm of period T, Iwishes to talkwith a subscriber Pn (not shown) connected to line Zn characterized byan instant tn of period T. The impulses received at point C will then betransmitted to the modulating gri-d M' yof distributor D2 by means of aconnecting circuit J1 which is operative only at the instant tm andwhich comprises an arrangement delaying the impulses by itu-tm if tn tmor T-tm-tn) if tntm. These impulses go from point .Cconnected on theother hand to the ground Aby a resistance R' and influence the intensityof the electron beam of the electronic distributor D2 at yinstant tnwhen this beam passes over electrode en. They generate therefore on lineZn impulses, the amplitude of which is determined by the microphonecurrents of the subscriber Pm. In the ,same Way the microphone currentsof subset Pn make the potential of point C Vary and the impulsesproduced undergo a delay of T- (tm-tm) if tn tm or tnt-tn if tm tnthrough a connecting circuit J2 which is made operative only at instantstn, to reach line lm. It will `be noted that the electrodes e ofdistributor D2 do not necessarily emit secondary electrons since thecommon electrode E is not used.

It is possible to simplify the circuit of Fig. 2 by using only onedistributor as shown in Fig. 3 in which the same elements are designatedby the same-reference numerals as 4in Fig. 2.

In the preceding schematics it has been assumed that the originalimpulses are delayed by a sufcient length of time to be transmitted atvthe appropriate instant to the other subscriber.

It is also possibleto reconstitute irst the original signals in theconnecting circuit and to only let go vportions of these signals throughan electronic gate which is opened at the appropriate moment. portionsof the signals going through are new impulses which are produced at theinstants characterizing the subscribers.

If it is assumed that the subscriber Am speaks, the potential ofelectrode em varies with corresponding fluctuations of the electron beamat instants tn and therefore produces potential fluctuations .ofelectrode en. These Variations react .on .the beam at instants tm andincrease or decrease .the amount .of the originel. fluctuetions of thepotentials of Belectrode em with a risl; of completely. distorting theoriginal currents.

Such an -eiect is avoided if use is made of two distributors, one forthe incoming and one for the outgoing signal; these distributors may beconnected to a .directional device such as, `for instance, balancedtransformers of the kind used in telephony, but this entails an increasein the equipment.

Another method is to use a compensating arrangement adapted to produceeach time the beam is modulated a potential .difference vedual and .ofopposite sign to the .potential variation of the collecting electrode.by .the variation of amplitude of the beam. In this case the ,signalsreceived from the subscribers are not modified but no ysignal isreceived at the exchange if the potential variations of .the .collectingelectrode are due to these variations of the electron beam.

In order to be able to establish several connections at the same timebetween :telephone sets connected to the Vdistributor a .certain numberof connecting circuits are necessary, all connected in parallel, lbutwhich are connected electrically to the. common equipment only atinstants t .of the cycle T which characterize respectively the differentsubscribers connected by this circuit.

Having thus described the method used .an embodiment will now bedescribed incorporating features Aof the invention shown in Fig. 4 ofthe drawings.

The embodiment may be divided into four principal sections:

(l) A group of subscriber lines shown schematically by bo-,X I, theselines being characterized by the position of their respective electrodese in the line distributor '263, i. e. by the instant ft corresponding tothese electrodes in a sweep cycle T.

(2) In box II the equipment common to all the subscriber lines and theconnecting circuits.

A(3) In bex II-I a connecting circuit comprising the calling junctionsJA and the answering junctions JR respectively associated `with speechcircuits BA and Ba. For the sake of clarity only one connecting .circuithas been shown in toto.

'(43 In box IV a register .circuit El and and a busy test circuit OC.

On this figure the following designations have been 4used -todiierentiate .between the elements of each of fthe four groups; theelements shown in box I are designa-ted by indexes beginning withnumeral l, those of box II lby indexes beginning with numeral 2 and soon vfor boxes III and IV. Gn theiother hand the velectronic gates havebeen designated 4by signs similarto those used for relayicontaots. Thearmatures Vare shown in their beacon rest positions; when va contact isshown closed the corresponding circuit is closed when n` impulse isapplied to the relay controlling this contact; when the contact is shownopen the c'orresponding circuit is interrupted when no impulse isapplied on the relay controlling this contact.

The shape of the impulses flowing through some conductors has been shownon the drawing with the indication of their position with regard to timein the sweep cycle, tm designates the characteristic instant of thecalling subscriber Am and tn the characteristic instant of the calledsubscriber When a subscriber Am lifts his receiver the cor respondingelectrode in the electronic distributor is at a given potential and ateach passage of the electron beam over this electrode an impulse appearsat point C. This impulses thus generated reach one of the pairs of freeconnecting circuits, e. g. circuits JA and JR. When circuit JA is busyit remains unoperative for all impulses whichA do not reach it atinstants im by lead 23|, but it re-` ceives the impulses arriving atinstants tm. The amplitude of these impulses is constant as long as thesubscriber does not usefhis dial or does not talk. Their frequency isequalto the number of revolutions per second of the electron beams andthey occurv at a time tm with respect to the origin of the sweep periodT.

Each connecting circuit is associated with a register circuit E I, E2and as soon as the exchange is ready to register the number of thecalled subscriber An, a signal is sent by means of impulses whichmodulate the electron beam at instants tm, i. e. when this beam passesover the electrode connected to subscriber Am. When the subscriber Amhears this signal, he dials a number. This causes variations ofpotentialof electrode em connected to subset Am andvdigits are receivedby the register in the form of 'a' sequence of impulses. These digitsignals are received in their 'origina form by the use of a low passlter,` The register is actuated according to the lnumber dialled andsends impulses at instants tn corresponding to the line of the calledsubscriber. If this called subscriber is free his ringer is operated, ifhe is busy the calling subscriber gets the busy tone.

When the called subscriber lifts his receiver the circuit is in talkingcondition and energizes talking circuits BA and Ba. When the callingsubscriber hangs up the circuit goes back to rest. The control of thecall remains therefore with the calling subscriber.

' Having now pointed out the main features of the operation of thecircuit, Fig. 4 will now be described in detail.

A number of subscriber telephone sets, one of which is shown at Am, areconnected to the secondary emission electrodes e of theV singledistributor 2i) corresponding to the distributor DI of Fig. 3. Theelectrodes@ are normally at a positive potential and the collectingelectrode E at zero potential. When the subscriber Am lifts his receiverhe closes a D. C. circuit comprising the centrai battery B, theindividual resistance RL and RL, the line and the subset Am. Theelectrode em of this line is then at a negative potential and when thesubscriber talks this potential fluctuates accordingly. The electronbeam F emitted by cathode K and which may be modulated by grid 2M,corresponding to electrode M of Figs. 1, 2, 3, passes through theopenings of collecting electrode E and strikes successively theelectrodes e which emit secondary electrons. The number of thesevelectrons which are collected by electrode E depends on the potentialdifference between e and E. These secondary electrons iiow in thecircuit: em, RL, B, R, C, E, and -induce a negative potential at pointC. Before the subscriber lifts his receiver the electrode e is positivewith respect to electrode E, very few secondary electrons beingvcollected by the common electrode E and the potential atn point C atinstant tm, which is characteristicr of subscriber Am, may be very low.When the subscriber lifts his receiver and thus brings electrode em to apotential lower than that of electrode' E many electrons may becollected by electrode E and the potential at point C may besubstantially lower vthan O, i. e. that negative impulses l take placeat point C at instants im, the amplitude of whichv varies when thesubscriber speaks.

A device 23 transforms the negative impulses 710 into positive impulses'l l. These impulses are sent simultaneously to all the connectingcircuits of the' exchange by a conductor 25! connected to a point A atthe output of device 23.

When subscriber Am is listening, beam F' generates impulses on electrodeem. In each subscriber line is provided a low pass filter comprising aninductance i and a condenser CL, so that only the amplitude variation ofthe impulses received in electrode em is transmitted to the line and tothe subscriber set Am. As long as the other subscriber does not speakthe intensity of the beam F is constant, the amplitude of the impulseson electrode em is constant and the subscriber Am does not receive anysignal. When the subscriber An speaks the beam is modulated and themodulating frequencies reach subset Am. To avoid an harmful reactionofthe beam modulation on the incoming signal and therefore on theoriginal potential of electrodes e, the device -23 is provided with acompensating device which may produce, under the control of thepotential variations of grid 204 of distributor 26, potential variationsequal and opposite to those induced by the-action of thebeam on thepotential of point C.

This compensating arrangement may comprise for instance a vacuum tube,one grid of which receives the incoming impulses from conductor 2l2 andthe other grid the outgoing impulses from conductor 22! by means of aresistance 235 of appropriate value so as to obtain the desiredcompensation. It is clear however that other compensating arrangementscan be used.

The operation of the connecting circuits will nowrbe explained. A device215i) can interrupt the direct connection between conductors 23! and 22by a potential applied to lead 244, but in this case the impulses 'ilfrom point A can still reach another connecting circuit by conductor 52if device ABill will let them go through. On the other hand the impulses7l may reach a point D if the circuit element 392 will let them pass,and the connecting circuit JAZ if the circuit element 3M will let thempass. The circuit elements 240, 301, 35H2, 308 are controlled bypotentials applied respectively on conductors 35, 34, 32 and 33 by thecontrol circuit 30B. Impulses l can pass through conductors 2222 and52and gate 387 to point D on conductor 3S, towards lter 40! throughconductors 3T and 38 and towards filter 352 by con-l ductor 3l.

If we assume that the rst connecting circuit JA is not busy, and that noother subscriber has lifted his receiver, the impulse l'l which followsthe unhooking of the receiver by subscriber Am goes through contact 240which isv at rest and ass-.amos

closed, .conductor 2152 and contact 302 which is also at rest andclosed, and it then .enters the control circuit 30B which operates. Itmay be noted that this impulse cannot reach the next connecting circuitbecause contact 361 is open.

The control circuit 3GB transforms all the impulses received `at point Dby conductor 3i into impulses transmitted respectively to conductors 32,33, 3d and v35 to operate this circuit, and to 36 for the control of thebusy test circuit OC. By conductor 32 an impulse l5 opens contact 302during the time interval separating two successive impulses tm comingfrom .subscriber Am, while, bi1-conductor 33, an impulse 813 closes contact 30! during the same time interval. An imx pulse T8, applied onconductor 34, closes contact till during a time interval equal to theduration of a channel impulse at the moment when the following tmimpulse must arrive. An impulse l, applied to conductors 35 and 241i,opens contact 255i! during the time interval when the following impulseof the same subscriber arrives.

After the passage of the Aiirst impulse '11 the fact that contact 3&2 isopen and contact 301 closed denotes the busy condition of the connectingcircuit .during the time interval separating two successive impulses atinstants tm, this interval being equal to T-At, where 'I' is theduration of a sweep cycle and At the duration of an impulse. At the endof this time interval and when the following impulse '11 arrives theimpulse .T9 from the control circuit 30D opens contact 2120 as explainedabove and prevents this impulse from disturbing another connectingcircuit. On the other hand the impulse '13 coming from the controlcircuit through conductor 34 closes contact 301 so that, at the outputof circuit 23, all the impulses 'l1 which follow the first impulse gothrough point A, conductor 232, conductor 52, contact 3Q?, conductor 39,point D and conductor 3i to reach the control circuit 3M. Each of theseimpulses causes, like the irst one, the operation of the control circuitand the transmission oi the various control impulses sent on conductors32, 33, 34 and 35. A steady condition is then reached in which eachimpulse 11 prepares the way for the next impulse '11 until thel callingsubscriber hangs up. At this m0- ment the control circuit does notreceive any more impulses l1, goes back to rest and frees the connectingcircuit.

If another subscriber calls while the first connecting circuit is busythe rst corresponding impulse '11 passes over contacts 241| and 301,conductor 243, etc. until the rst free connecting circuit is reached,since the impulses 'l5 on conductor 32, and impulses 88 on conductor 33,produced by impulses 'i1 at instant tm, open contact 3BE and closecontact 391, sending the impulse 'l1' oi the new calling subscriber tothe nent connecting circuits.

impulses li of subscriber Am cannot disturb other connecting circuitssince contact 240 and contacts such as 3c? of these circuits are openinstants tm.

In the case, when the first connecting circuit becomes free, theimpulses corresponding to the subscribers connected to the otherconnecting circuits cannot reach this rst circuit since contacts 251! isopen at each instant characterizing these subscribers, but a new callwill generate impulses 'lip at an instant of the sweep cycle which isdifferent, and these impulses reach contact 240 when it is closed andwilll therefore be able to; seize the first connecting circuit.

4In case of overload, i. e. when all the connecte ing circuits arelbusy, the impulses corresponding to the last subscriber will pass'through conductor 2M, contact M51, conductor 2&3, contact Sti of thenext connecting circuit and so on until the last connecting circuit.These impulsos are lost until a calling subscriber hangs up and aconnecting circuit is thus made free. The impulses oi the waiting callthen seize the circuit.

Junctions .la are used to reach the called subscriber but instead oibeing controlled by impulses coming from the calling subscriber, theyare controlled by impulses Bil) from the register as will be described.below.

The electronic gate 25.9 is also closed at the instant characterizingthe called subscriber by means of impulses sentori conductors i3 and 21Mas will be explained below. In this way a connecting circuit cannot befound busy by the impulses from the called subscriber when he lifts hisreceiver.

After the seizure of the connecting circuit JA by subscriber Am theimpulses li are directed to register EI associated with this circuit.Since this register is at rest contact 2l of one of the step-by-stepswitches is closed. On the other hand contacts 313 are closed and thetone tells the calling subscriber that he may begin to dial through thefollowing circuit: tone generator 24, Contact 421, contact 318, contact320, conductor 221, grid 20d. Contact 321! is closed at instant tm underthe control of impulse 'l1 coming from conductor 53. This tone changesthe potential of grid 20d and modulates the electron beam F of theelectronic distributor 2t. When the calling subscriber hears his tone,he can dial and send the dialling signals by means of impulses '1lthrough conductors 232, 52, contact 39! and conductors 31 and 38. Theseimpulses reach lter 4131 where the original signals are regenerated.

These signals, corresponding for instance to a three figure numberoperate in a 1anown manner the step-by-step selector switches orregister El, which assume positions corresponding to the digits sentbythe subscriber. As soon as the rstdigit has caused vthe rststep-by-step switch to move brush 21 interrupts the tone circuit when itleaves its rest position. The step-bystep switches adjust a number ofretardation lines, three in the case of the gure, 23, 112:3, 425 bymeans oi: circuit 412, 413, di@ respectively. After the reception of thelast digit, relay m3 of the register operates. This relay operates withsome delay, a few tens of milliseconds, and closes a number of circuitswhich remain operated while the register is busy. The adjustment of thisrelay M3 is such that its Contact 432 is closed a few millisecondsbefore contact 434 opens and contacts 433i and 4555 are closed aftercontact 434 is opened. By the adjustment of the retardation linescorresponding to the positions imparted to the step-bymstep switches bythe digit sent by subscriber Am impulses tti) are produced at instantstu corresponding to the called subscriber.

It is possible to use a set of retardation lines connected in series anddesigned with the same degree of precision for the long and shortdelays, but it is preferably to use the arrangement shown in the drawingwhich operates as follows:

A circuit 21 supplies the registers with a certain number of diierentimpulses corresponding respectively to 10, 100, 1000 time intervals tseparating the scanning of two successive lines. In the particular caseconsidered here it is assumed that a subscriber is characterized by'athree gure number and that circuit 2l' supplies three kinds of impulses:impulses L.llill on conductor Lli which are generated at the beginningof each sweep cycle at intervals of 1000 impulses Litt? applied to aconductor Ll at intervals equal to 100 t at a rate of iten per sweepcycle, and impulses LIC applied to conductor Lib and occurring 100 timesper 'cycle at intervals of iO-Xt. By means of re-I tardation lines 423,424, 425 respectively it is possible to retard an impulse of anypredetermined amount. Let, for example 532 be the retardation line 423controlled by the first stepby-step switch. The number of sections o theretardation. line 423 in circuit depends on the position assumed by thisstep-by-step switch and corresponding to the number` of hundreds -in thecalled number, each section of retarda- .tion line 423 being one tenthof the duration of the sweep cycle i. e. 10015. In the example chosenthe impulse Liii Will be delayed by .5 100 t at the output of theretardation' line 423. Let Ti be this impulse. This impulse is ,thensent to circuit l associated with the second step-by-step selector andcontrols an electron gate which is shown on the gure as contact 4i 3. Ofall the impulses Lilli! present on `conductor Lit@ only the one whicharrives at `the same time as Ti will pass.

This selected impulse Lili@ will be delayed by the sections of .theretardation line 424 which are put in circuit by the second step-by-stepswitch in the position in which it is at this time. In the examplechosen this delay Will be 3 10 t and an impulse T2 is thus obtained atinstant Of all the impulses LIU arriving from conductor LH) and appliedto circuit H4 only the .one arrives at the same time as impulse T2 .willpass, i. Llil thus selected will in its turn be delayed by e. at instant53M. The impulse the section of line 42 putin circuit by the thirdstep-by-step switch. In the example described the last delaywill be 2XLTherefore yWe obtain at the output of register E2 a nal impulse 84Ddelayed by 532i with respect to the beginning of the sweep cycle. Thissuccession of selections is repeated at each sweep cycle of thedistributor.

Thus impulses 84D are received on conductor 54 at instants tncorresponding to the kcalled .at these moments.

If the called subscriber is already busy impulses are present on line244 at instants tn which, by conductor 5i), contact 4331- and conductor4t reach relay 3% whose contact corresponding to conductor 46 is closedat instants tn as seen above. -These impulses tn from conductors 244 and5E! pass through contact' 4331-, contact 368, conductor 4t, contact 434,which is still closed, and conductor 55 to the busy test circuit OC. Ithas been seen that contacts 433i" and 434 of relay 403 are open with adelay 'of a few milliseconds after the closing of contact 435 so as topermit the generation of impulse 'subscriber and these impulses closecontact 398 60 :84D and the operation of the device 308.

. the eventual 'seizure of another This delay permits also a busy testof the called sub- ;scriber by the change brought about in the conditionof circuit 416A by impulses tu. Arrangement l4|6 is av flip flopYcircuit Well known in the art. In the case when subscriber An is busy,the arrangement 456 swings, closes contact 4H, opens `contactlii andchanges by conductor 41 the condition of circuit which opens luntil thecorresponding JR circuit is free. Moreover the device M6 opens contact422 to avoid mixing the busy tone withv the ring back tone.

4It follows that in vthe busy condition relay S -is `operated by localimpulses 84D applied to it by conductor 54 only for a short timeinterval, tomake possible the busy test. After this test locking ofdevice EU8 by the impulses sent on conductor 41! prevent the impulsestu. from the vbusy subscriber, which arrive from conductor 232, throughconductor 5I and contact 433i, to reachcircuit BR by conductor 40.Therefore if Ythe calling subscriber does not hang up he'cannot overhearthe conversation in which the subscriber he has called is taking part.

The busy tone Afrom generator 2t may then pass over contact 4i?,Acontact titl and contact 32e, which are closed at instants tm to themodulating electrode 264 of distributor 2li. Subscriber Am thereforegets the busy tone; he may then hang up and the circuits go back torest.

4When the subscriber hangs up the control circuit S does not receive anylonger impulses l and cancels therefore impulses lil on contact Stil',which opens. The Contact of relay 4M is closed and brings the registercircuit to rest by the 4.signals sent by generator 2l'. At the same timeby conductor Se the control circuit lili brings the busy circuit 4Mvtoits rest posilon.

In the case when the called subscriber is free, circuit 4I6 remains inthe same condition since there are not impulses tu on the circuitcomprising conductor 244, conductor contacts 4331' of relay 4813,conductor 49, vclosed contact 38, conductor 4t, contact 434' of relay lland conductor 55. Contact 4H is therefore open and contactv sit remainsclosed. Inv this way the busy tone is not applied to circuit BR. As soonasrelay 403 has closed its contacts 435, 4331i, 43h

the impulses 84D from retardationlnes 423, 424 and 42e' pass overcontact 435 and contact 4de and are applied to conductor 322 and closecontact ssl at instants tu. The closure of contact 4t! enables theringing current from generator 2: to pass. over contacts 422 and 3i@ todevices 32u and 32|. Since contact 32B is closed at instents tm thecalling subscriber gets the ringing bachI tone. Onthe vother handclosure of contact 435 applies impulses i34Don conductor 45. '.Lneseimpulses are applied to device elle which changes their sign and givesout positiveimvpulses 36D applied to conductor 244 by conductor 43. Thepresence of these impulses on conductor 244 denotes-that subscriber Anis busy. If before subscriber As lifts his receiver, another subscribercalls him, this other subscriber will'rece1ve only the busy tone asexplained previously. Moreover the impulses 86D applied to conductors244 close the gate 240 at instants tn and prevent connec cuit byimpulses tn.V4 tmg mr Vl/'hen the called subscriber liftshis receiver,impulses tu appear at point A and open contacts svpuylihe `OIICPVVIIgcircuit: point A, conductors.

decanos Sitz and I, contact 33t of relay 403, conductor it, contact 353,closed at instants tu by impulses SAD of conductor 54, conductorl.Contact BIS disconnects at this. moment the ringing circuit fromcircuits BA and BR Which are then in talking condition.

Circuit B n., corresponding to subscriber An, is controlled by impulses84D which come from the `retardation lines through contact 435, contactH9 and conductor'322 and which close contact 32! at instants tu.

Impulses tm modulated by subscriber Am are transmitted by point A,conductor 232, conductor 52, closed contact 301. and conductor 31, tofilter 352V of circuit BA. This filter gives back the original voicecurrent, and on conductor 22-I, after contact 3.21, of modulatedimpulses at instante. tn.

Thus has been achievedthe connection of subscriber Am to subscriber An.

The` Ba circuit. of subscriber Agi is unblocked by impulsesat instantstm. Theseimpulsescome from point Dthrough conductor 53, a device 3I'9changes their sign and they close contact 3-20 at instants im. Frompoint A, conductors 232'- and al, contactltt, conductor 4-9, contact308, conductor du, and conductorY 405 the impulsesf of subscriber An,modulated at instants tn are transvoice frequency While, after contact320, modulated.y impulsesappear on conductor 22I at instantstm. Thus isalso achieved the connection of .subscriber An to subscriber Am.

As it; has been shown previously, the call is underv the. control. of.the calling subscriber. If this subscriber hangs.. up thecontrol-circuit 300 no longerreceives impulses 'Ml and is freed asexplained above; on the contrary if the called subscriber hangs up,control impulses 84D are Still suppliedto device v-byrconductor 54 andff theconnecting circuitv does not go back to rest.

Having., thus described` the invention inl its broad. aspects. Weshallnow describe in detail an embodimentinrelation. toEigs. 5, G'and'lassembledas shown on. Fig. 8

Tntheselgures, the conductors connected to ahigh potential isource. havebeen terminated `by anarrow..

The,A subscriber. setiwhich comprises aY hook, a repe'iyer,` a,transmitter, a dial and1 a ringer is schematicallylshown by a contactIl,y reminding that ughenthe subscriber has lifted his receiver a,circuit, has` been 'completed'. in his sub-set.y

lachsubscriber Yloop comprises individual resistancesIIIiand. II2-,corresponding to resistances andRiL oflFig. 4, anda ltercomprisinductance. coil .I I3, and-1a condenser I I4; l?, .9" grounded`common.batteries-I I5 (Bonv Fig. 4l argeles .provided The electronbeamA distributorli.. comprising a, cr'nnion electrodeS, electrodes, .I0.1 connected4 lrespetively toE rthe tel ephonelines, deflectineDlfSDiDi l, DI?, Dlalid ampdulatins grid 2&4, has been described pryiously. Delecting ple-ies D i 01. D! l, D12, D13, 24,19; @Densetsu.tothe control circuit 2l which' comprises akcircuit FI which suppliesythe sweep frequencyof,l the elec; tron beam followed, bya series offrequency mule tipliers, tWoin this example, and respectively designatedby numerals IDFI and IO0FI, and by a dephasing circuit DF. Three impulsegenerators Iltiit, Litt, I.I0 respectively controlledby cir- Quit: .ELland. these multipliers supply the register Qlillih. throughlines LI000,LIUUfLYIUKrespectlv-elm with-threekindsoi impulses, the duration andvpurpose of which have been explained previously.

Generators 2i, 25, 26, 21 supply respectively the dialing tone, theringing back tone, busy tone, and release impulses, and are connected totheir respective circuits in the usual manner by means of transformersT3, T4, T5, Tt respectively.

When subscriber Am lifts his receiver negative impulses 'lil coming fromelectrode 203 ow through a tube V23 corresponding to device 23 of Fig.4. LThe cathode of this tube is brought to an appropriate potential by aresistance RI; its grid is provided with a leak resistance R2 and itsanode with a load resistance R3 connected to the high potential. Thecompensating arrangement mentioned. above comprises a potc-rntioirleterl235 Which biases a second grid of tube V2.3.

Impulses l0 appear at the output of this tube, as. explained previously,in the form of positive impulses il, which are applied by a condenser C3to. point A.

The iirst positive impulse li which appears in A, atinstant tm after thereceiver of subset Am hasbeen unhooked, is applied simultaneously on thegrids of tubes VG (Fis. 5) and Vl (Fig. 6) and, by conductors 232 and5i, to contact 1333i? ofy relay 303 (Fig. 7). A .bias resistance Rt isprovided in the grid circuit oi tube Vi. Tube VG@ common to all theconnecting circuits, is a secondary. emission tube with two catho-des Kland K2 Irespectively to which appropriate biassing potentials areappliedby resistances RS and Rl respectively, The grid of tube VG is providedwith a leak resistance R5. This tube supplies, byy conductor 2M andlcondenser Cil, positive ampliiied impulses lpin phase with impulses ll.Normally, tube VG is blocked by the positive potential applied toitscathode Ki. The rst positive impulse 'il passes through, but the tube isblocked to the folloivingimpulses 'li generated at instants tm by thepositive impulses 'iii suppliedby tube V5 through conductors 35, 55 and2,44, and at` instants tm by impulses tD from tube V9 through conductor43, condenser C5 and conductor 2131i. The impulse 72 produced in theoutputA of tube VG by the first impulse Il is directed to point G andapplied to the grid of tube VI which corresponds to device 30! of Fig.4. This tube Vl is a secondary emission tube. Its -rst cathode isconnected to the cathode of tubey Vtand its second cathode. K3 isconnected to the high potential battery through a resistance Ru and toconductor 243 through acoupling condenser C5. This tube lets pass4 therst impulses or" other calls in case the irst connecting circuit isIbusy as will be seenlater.

ivve assume that the first connecting circuit is busy i. e. that tube V2is blocked (this `tube V2 corresponds to contact 302 of Fig.v 4), tubevVi being unbioclred, the iii-st impulse '52u of another cali, inthevoutput or" tube VG, follows conductor 2R33 until the iirst freeconnecting circuit; When this impulse finds a reeconnecting circuit itis applied to a tube V2 of this circuit, thev grid of which is providedwith a leak resistance Re and av plateresstanceRi0. When lat rest tubeV2 iis blocked by the positive potential applied on its cathode, for.instance byv abatteryBiZ, andit is unblocked by impulsev l2. Thisimpulse appears atpoint D as a negative impulse i3, after it has passed`through a coupling v condenser Cl. This impulse F3 causes the controlcircuit 3&9, to operate, which comprises tubes Vil, Vi and V6.

'Il ie grid of tube V3 isprovided wtha Yresistance RII Aand its anodewith a load resistance RI 2.' By condenser C8 the 'anode oftube-V3 isconnected to the control grid of tube V4 --which is provided with aresistance RI3y connected to the high potential. The screen grid of.this tube is connected, on the other hand, -to the vhigh potential by aresistance RI4 and to the ground by a condenser C9. The suppressorgridis fconnected to the cathode which isp-biassed by a resistance RI5, andthe anode connected to the high potential by a resistanceA RI 6. Thecathode of tube V5 is connected to the ground :by a resistance RI'I andto conductor 35 -to transmit irnpulses 19, its grid is negativelybiassed for example by a battery B3 and a resistance RIB, `and its anodeconnected to the high potential by a resistance RIB. Tube V6 is biassedby means vof a battery B4 and a resistance R20.. 1 f,

'Ihe control circuit 300 sends on conductor 32 a square shaped signalthe leadingA edge lof which coincides with the end of the rst impulse 13and the rear edge with the front edge of the following impulse 13. Thissquare shaped signal, which is generated as explained below, is used intwo different Ways, as explained below. e

In the, case when the duration of the impulse 15 is not altered thisimpulse appearing on the cathode of tube V4 across the terminals ofresistance RI5 is applied to the grid of the tube V2, and blocks this.tube during rthe interval ofr time separating two consecutive impulses12. It blocks therefore tube V2 vfor all the impulses which comefromother subscribers and places the connecting circuit inlbusyvcondition for the call which is characterized by instant tm. f Thesame signal is applied to the grid of ltube VE whose biassing potentialis common to that of tube VI. This bias is 'such that the tube VI @isbeyond its cut-off point in the absence of impulses. Thus the cathode oftube VI is brought to a potential such that it isoperative during thetime interval separating `two consecutive im pulses, and this enablesthe impulses ofl other calls to pass through to line 243. Y

In the other alternative the negative impulse 15 is sent through a timeconstant circuit C-Iil, RIS which gives two impulses, a negative impulse11a and a positive impulse 11b. A battery B3, for example, applies totube V5 a threshold Apotential which eliminates impulse 11a; the impulse11b, produced at the end of impulse 15, is clipped and gives in theoutput of the tube V5 two short impulses, a negative impulse 1.8 on theplate of rthis tube and applied to conductor 34,` and a positive impulse15 on the cathode of this tube, applied to conductor 35 and to conductor244 over conductor 50. This impulse 19 is thus applied tothe cathode KIof tube VG and makes the tube nonconductive for the impulses 1I whichfollow the rst impulse. In this way all the following impulses 1I of thesame calling subscriber will go through lines 232 and 52 to tube V1.

The cathode of tube V1 is polarized beyond its cut-01T, for instance bymeans ofV a battery B5 and its grid is connected to a leak resistanceR4.

The negative impulse 18 is applied to the cathode of this tube and makesit conductive, this tube thus prepares the way for the impulses 1Icoming from the same calling subscriber and which cannot go through tubeVG. Thus are obtained in the output of tube V1v negative impulses 13Dwhich are applied yby a condenser CI I to point D by conductor 39 and tothe control circuit 350. A v

vThe operation of tubes V3and V4 which gives the square shaped signal 15of duration (T-t) will now be examined. .'Ihe` rst-negativelmpulse 13Von conductor 31| at the' output" of tube V2 is appledto thegrid of tubeV3. In the absence of impulses a plate current ilows in this tube with acorresponding potential vdrop in the anode resistance RI2 4so that theplate potential of tube V3 is brought to a very low value UI. Thenegative impulse 13 which has an amplitudesuperior to the cut-offpotential of tube V3. blocks this. tube, annuls the plate current andbrings the plate potential to a potentialUZ Iabout of the same magnitudeas that of the high potential source. Tube V3 goes from lpotential UI,in the Iabsence of impulses, to potential U2 during the passage of theimpulse. Condenser C8, charged during the impulse, discharges'exponentally and gives an impulse 14 between two successive impulses13. The control grid .of tube V4 is connected to the high potentialsource through a resistance RI3 or" very high value so as to be broughtto a positive potential which is only slightly higher than the cathodepotential. When one positive impulse from tube V3 isfappled to the platea of condenser C8, this plate is brought to potential U2 and thecondenser charged through resistances RIZ,` RIE. The positive potentialof the grid of tube V4 increases slightly by reason of the chargingcurrent owing through resistance RI5 and gridcathode resistance of tubeV4. a

At the end' of the'impulse the plate a ofy condenser VC3 is againpotential UI. The decrease in the potential UZ-UI is transmitted to thegrid of tube V4 which is in the form of a negative potential UI--U2.Tube V4, which during the passage of the impulse had a normal platecurrent, is suddenly blocked and condenser C8 discharges slowly throughthe grid leak resistance RIS, resistance RI2 and'internal resistance V3`in parallel. The increase of the grid potential of tube V4 depends on atime constant which is substantially equal to the product of thecapacity of condenser C8 by `resistance RIS. As soon as this potentialreaches the cut-01T potential of tube V4 this tube is unblocked and aplate Vcurrent flows corresponding to the end of the square shapedAsignal.' The steepness of the second edge of this square` shaped signaldepends on the length of time nec-' essary for the grid potential toincrease from the cut-olf potential to the cathode potential; theshorter this time interval the steeper is this second edge. It isadvantageous thatl the Idischarge be rapid when the grid potential iszero with respect'te the cathode. To this effect resistance RIS isconnected to a high positive po tenti-al; and .an extremely small gridcurrent is sucient to maintain its potential at the potential of thecathode.

Thus on the cathode and plate of tube V4 long impulses 15 and 16 ofduration (T-t) are obtained. 1

At the output of adecoupling circuit comprising a resist-ance R2I and `acondenser CI 2 the impulses 1'6 go throughk con-denser CI3` and supply aD. C. potential on conductor 36 during the Whole time that controlcircuit 300 is in the busy condition. As soon as subscriber Am hangs upthis potential disappears, thus bringing the iiip-op circuit VIM-Vith(Figure 7) to rest as will be seen later. The connecting circuit beingthus seized subscriber Am gets the `dia-lling tone by the rest contactof step-by-step sWitch'PI of the register (Fig. 7) by the followingcircuit: Generator 24 (Fig. 5) common to all circuits, transformer T3,conductor 5B, rest contact of switch.P"l` (Fig. 7) .conductor 40,-grids` of.. the

new "s doublel triode Vfl-8:1 1ig.. 6);. In: the: anode: cir"- cuitsv ofthis: tube are provided two= resistances R22 and R23y respectivelyi,"vand.- 'ai resistanceA 24 in the grid circuit. The dialling tonev appliedto. the tvv/oy grids of tube VI'B, is sent. through condensers Clit andC45 respectively to'.I points b' and` d, which are grounded; through..resistance R251 and R21?v respectait/rely.Y This tiene isi there`F fore:applied. to; the control; grids of tubes. V20 and VEL. Tube. V2@ alonecan be madel opere ativeby impulses 13 coming. from point D* onconductor 53' Which are amplified'. and reversed in. polarityby tube VISWhich. operates as aI limiter: An appropriate potential. is; applied te.the cathode'. of this. tube by ay resistance: Ril-'1.y In' its grid:circuit. is provideda resistance: R28; and an anode. resistance. R293 isconnectent between the'. anode and. the source of anode potential; Theimpulses: amplified by thisttubeflow through condenser Clt. -andapply apositivepotential on the. grid; of tube V2 ll: which grid is connectedto ground through;` resistance R39. @n the other hand. the cathodepotential; of tube: V20 isv sup'- plied by aresi'stance. R311 and: a.battery B104 for example. Its anode is connectedto agrounded resista-nceR32'. andto theano'de. sour-ce. for tube V2 lf over conductor. 2 2i;

Tube V20. being thus'. made conductive.V by irnpulses 13 transmits tosubscriber Am the iin'- pulses. modulated. by. the dialling: tonethrough conductor 22l, the" modulating grid 294,. the electron beam.of.' the distributor'z? and the. subscriber line; When subscriber Amhearsthis tone lie' can.4 use his dial' for. sending. digits. whichwill. increasethe amplitude of the positive irnn pulses". at pointAbyaratio of1 three halves.

Itvvill be assumed: that4 asepreviously the number of.' the calledsubscriber has three digits; The delayed impulses: 1B from anode V5change at instants im the potential of tube V1. from; a restpotentialtothe cut-oilpotential;v Theiam'- plitude modulated" impulsesy 1T!!applied? to the grid of' this tube V1 give rise to a mean plate currentand therefore decrease the value'ofthe potential between termina-lsf ofresistance: R33 following a. resistance R34 and condenser CH; ResistanceR33 is on-.tlie other-.hand connected to the ground through a`resistance R35 and; to the high potential through resistan'cesR36n andR31.' Across terminals of resistance Relnegae tive impulses. arecollected which.. are transmitted by conductors 31% -andt tosfilter-401, Fig. 7; This filter: supplies therefore negative se'u le'cting.signals whichv block the grid of ampli ner tube Vi l' Whichisconnected'totheground throughA a resistance R38. The cathode of thistube is connectedtoit'he ground'by a-.resistance R39 and its `anodecontrols a relayilltz. through a resistance R40; Each time this-tubeisblocked bythe negative :selection signals relay'402. falls back. andbeats in synchronism with thesefsianals;A The" electro-magnet orstep-by-step switch PI is operated by the' followingI circuit: ground;`Winding of electroemagn'et Pl rest; contaetP'Z, rest' Contact oi relay482- and battery; At? the end of the rst' signal: relay 402 operatesagain and the electro-magnet.' falls back; selector Pl,- moves one step.Thisy procedureV is repeated for every signal.

After thexrst digit'has been sent, which ain-the example considered isthe hundreds,- slowoperatingtand. releasing relay 4&5; whichhaslnot Ihadthe` time to operate duringthe passage lof brush of Bf I: over the.terminals, operates. by the. fol-- lowngcircuit: high tension,IresistanceRMfl ter.-l mihalzP!" l., Windixxgfoflrelay 40S/andthefgroundi Ey#v itsl make` eiinactl 4closes the circuit of theelectromagnet of sWit'chPZwhich, for the second set of? signals,vcorresponding Vto the tens, moves in the same way' asY step-by-stepswitch. Pl'. Furthermore'si-nceswitcli P'I' has left its restpositionthere i'snofdiallingtone,

When the line' linder- PZ sto-psat the end of the. second set ofAsignals" on the terminals corresponding' to the? digitsV sent bysubscriber Am,

, relay 4051 i's short circuitedby the row of. terminals P"""2 asis'een'o'nthe drawing., This relay therefore fallsback and pre-pares by itsrest contact a steppingr circuiti for the Stepby-step switch P3 whichyoperates-under the contrclnof impulsesv corresponding totheunitsasfollows: ground, electroemag'ne'tPSf, temiinal row P72, rest Contact.or" relay 405, restcentaet of relay 002 andthe batteryi.

After the reoeptionof they third digit and the stopping' of! line finder"et: on the. terminali corn respondingto this digit,.thefnumber of thecalled subscriber has beenregistered. At this moment slow opera-ting`relay 403 operateslover the following circuit; ground, terminal row Pmewinding of'relay 403?, resistance R02, high potential. This relay"fulfils during thef busyA test several duties which] Willi be described'later. It is clear that although `a; teleplfiorlerelayl has beendescribed it is possible' to`4 use ani electronicdevice for carrying outthe functionsof thisrlelayf03.v

The' stoppingof st'ep-'bystep switches'v Pi, P2. P0.' on some offtheirrespective terminals character-iz'esy the called n'untdoer.A The zero.yterminal corresponds to*v therestfpos'ition' and'V the eleventhterminaisto digitsero.- The terminals of .bank 4 of-` eachs'teo-byistepl switch are respectively connectedtoretardationlines LR I,ILLR2, LRS.

At the end of the-conversation. the returni to the restposition: of tliethree step-byestep switches is controlledby relay-'00e and tube V10;Byits makeioontact relayllifconnects the brush P3 to the. common-line51" which supplies permanently to alll the connecting circuits a poten-vtialy interrupted alittle'. rhythrn of the numbering impulses, a'nd-AWliielif is'tv supplied byreturn rto rest impulse generatorilf throughltransf-orme'r'Tt.

At the end'Y of the conversationv the disappear ance of impulses 1li:at. the inputo.. tube V1 by conductors 231i"Y and 52, through couplingcondenser C`l'8`, causesa change in the potential of tube V10.Tliecatliodefof this tube'isat agiven potential witlfirespecti te thegroundbyf means of resistances- R35; 3S.; 315' (Figure 6l. rihe passageolif impulsesl H1 througnvlf had given rise to a mean plate current andtherefore toa` potential drop betWeenpointslPand on resistances-Rl-l,y:31, andthe-gridoftube'fVlD ,vais-thus biassedbeyond: cut-01T' andrelay-'4001 add-rest.v The eliminat'idnlof impulses Tlblockstube V-Tgdiminishes the'- potentiell at'pointlPfan'd `brings back the' grid oftube VIDtoitsnormalbia'ssin'gfpotentialA The anode of' this tube istconnected to the tension" through a= resistance' R43? and the winding ofrelay'40111l Whe-n-tibeiVlUoperates relay 404 is excited;y The'generator"21 then sends 4through conductor Bland4 the make contact ofthis' relay404 impulses thecire'iiit orso/itch P3 which starts rotatingg'and. stopsas soon'as .itgbrush 1:"3 reachesthe restposition, since theoperatingcircuit of` electro-magfnetv is then open; At this moment.impulses are sent byrest- ContactI Pme to brush P"2 which also goes backto rest.r and controls.L the same Way;v the. heme return of switch EL.Thereturn to the* home position :of switchP1:I frees theregister- Ioriwhew calltsice resistance Rit andin itsgrid uit -p idear aresistaneeR45.. The imhulsesLla ar hee f tive.; they flow.- through tubo VI 2andappeara positive polarity in the anodel ci tube whieh is. oonheoterlto the h1 through a resistance Ritt and; to the retardation line LRiIthrough a.4 connecting.; condenser Q2u. This retardation line LR Ivcorresponds tothe 17 une.v dress.. digit of the calledk subscriber and;nected to terminals El, :1V of. the stepdoyssten, switohri- AseXblainedprev-iouslythe deiayi l trofiuoed; by eaeh portion ofk the lineoomsris between two. sueoessive terminals` otthe step lstepsyvitoh..oalled delay unit, is @ouai toftheitime interval -talreubythe. electron` beam or the elise tributorit to. sean a hundredterrrlirials.v of the distributor. if the `eteb-byfsten switoh h. s,.-stonsed on they othA teriiiirutl,l the impulseline LBA; will correspondto the nth' impulse the hundreds generator Lleill, tho origin impulse ofthe number of the called subscrib to the grid. of. tube Vitt.. theother. is ooriueoteol to liheh ittwhioh treuishiits t pulses -spacedanart by ten timeintonvalsfi are-produsse by,.- eeherator litt.l-he-.oathoti this tube is brought. to positive; uoteutialf respect.tothev grid. byy means.. for exampleo-` a. battery B- inserieswith aresister-recite?. No molly this-tubo is bloclieuby the hositive poteri f2irelied, toy its. oathode and.. does not let the posi tive. impulsesLitt so through. but when it ree ceives on its rst grid the positiveirnpnlso.from;l retardation libe .LRt it becomes oouduetive and ouitsoathode appears. a positive.imnulsefdceto theimoulse Litt which wasthou abulied! to. second,A grid. rlhis impulse is applied @Q1-hg youtardatioh line. LRQ. through; a.- eouolihs. Condenser C132... At. theoutput of thisfretartlatioh line.`l delayed impulse is applied; .o the.arid et al tube. Vit provided with a leak resistance R50., Thepositiveimpulses Lltare anbliecito, as eeo d.- of this tube. ltsoathodeis br-oushtto anos/thuepotentiai with ieSPeot to theerid, for examine.means or a battery B12... ahdits plate connected to the hier; tensionthrousha loadresistaneer Whena positive.k impulseirom.theretaidatiohline. LR21 makes this. tube conductive a negative. ithepulse appears on. its plate due toY the positive-iho-A ioulse Liuapplied at this moment to. its seeohd.- grid. This impulse is sent tothe retardation line,

by a, coupling condenser (333,.I

Finally aA negativer impulse Stil) iS. obtained. on. conductor 51 Whichis delayed' with respect toJ the origin of time by an amountcorresponding toL the delay character-isms. the called subscriber- Itmay be noted that. retardation lines a ,di LR?. may have only aliruited.number. of. settlers. and may slightly distortv the impulses. Withoutadverse eiieot, but the *retardation linelliRg,A which. imparts to theteus. impulsefrom lineLtt.- .afilelayfy characterizing. the units rnust`not distort the@ pulses., it musttherefore behroviiied-iwitheiently highnumber otseotiohs to. iuliil this eche dition.

ases.

Quthe other.' hand although. iu lthe exampledescr-ibedaregister;hasloeenl shown with step-bye. steptswitches, any other kind`of register, electro-A magnetic or electronic, may be, used. it is alsovclear that other kinds' of; delay devices may bey used withoutdeparting-y ffrom. the spirit of the in.-

vention. n

The closure. of contact $32 of relay li, takes place Very shortly,before; thev operation of` the other contacts, and this delay makes itpossible tov control. whether, the called subscriber is free` or busy.When the,l called subscriber` is busy this condition is characterised bythe presence on` linel 2M. of positive-,impulses 'HSD at instantstn,supplied. by the. cathode of the tube Vb of the oon-` necting circuit.associated with the subscriber through a,V coupling. condenser C2i andYc onduo.. tors andl 5t of this circuit as mentioned above. Theseimpulses 'I QDfiow through conductor 5t. of tho. connecting circuittaken unv by the calling subscriber Am. and are appliedv by Contact.t3/r. of relayk t3 andconductor eiitothegrid of; tube` WL corresponds tothearrangement 39S of Fig. 4. The cathode oitube VS receives throughconductor 54, the, local negative impulses Sill) fromthe retar-dationkline LR3'. On the other handv it Vis brought tov a positive potential bya battery El, for example through a resistanoe RSI,vv so, thatthe-impulses MD unblock this` tuloe.

Moreover the grid of tubeV. is, at a potential U3:

determined by the arrangement-Vltmviiib and; applied to it by conductoru? and resistance Rii as shownlaterfon. This potentiai is such that theimpulse. WD. at instants tn, muy-'HOW through tube V8 and leave. it thelgorinV of negative impulse- D on conductor et through a coupling con.-

deriser G22- Theseimpulses 85D are applied by contact 43u. oi; relayl4.63. to. the eridoi. a tube VitbW-hioh isa; part. ofthe uib-ouuirouit tl t of. Fis. t, which, at. this moment, is,.-i.u the free4 oohditiuu-This ooi1- dition isoharaoteriaed by the above; mentioned. potehtial.-U3: which. biasses. normally th@ sirio oftube Vith. A. nlateour eht howstherefore iu this tube. which, iholuoesbetweeuthe grid ahoi Cathodeoftube Vita a. potential fiiierenee hierher than the. cut-ofi uoteutialof. thistuibe. The tiro. tubes Vla and Vlia biassed in the saine Way aretherefore. bloelseft The negative impulses 35D applied totheeridfoiftube Vith. @hause its polar-i- Zatioii in such a. Way thatthe. stiel.- eoes.. from notehtial Ut. denoting-a free condition., to.b0- tential U4 denoting a busy condition. ljhis poteutial` Uil uisblaoesthepotehtialoi tube Vith and Yl'lh. beyond th. outfoilt noiut andbloolss The Hin-lioiioirouit. is of any known type. 'it

may` Comprise two tubes Vita and: 'Wto the` ahodes of. Whtuh. arenrovirietiV with load resistancesRS and Rii respectively connected intawell-known manner tothesrid. of the otherv tubel through resistahoes R55ahelfRttfahd .botentieni-l etersdtl` and R58... These gridsare-alsoprovided with,A leak. resistahees. Rit and littl respectively-Thefoathodesare blastedih.'-paralleh for exaihbie by means. of; abatteryBt.; also. l supply abiassinsnotential to the doublezltriofle tube Vil;-The anode et" tube Vl-'lo is oohheatedz to a load resistauoeRl.

when afterv this ohahee ih the condition, of the:

hip-lion oireuit relay 'ittoherates its, est: eentaots 4i@ aud 433.1'and oioses its et contacts stants in which flow on line 5| fromconductor 232, are applied by contact 433i to the grid of tube V8, butdo not go through since the potential of this grid has gone frompotential U3 to U4. Thus, no current ilows in the resistances R62 andR53 of the plate circuit of tube V8 and resistances R64, 65 and 66connected between the ground and the high potential act as apotentiometer bringing the grid of tube V8 to its cut-o" potential.

Tube V|8 is thus at its cut-off potential and receives on its grid thebusy tone from generator 26 and transformer T5 conductor 55, tube Vl1aand a coupling condenser C24. The busy tone is thus applied to the grid`172|) of tube V20 which is made conductive as explained above by theimpulses 13D coming from tubes V1 and VI 9. This busy tone is sent tothe calling subscriber Am by conductor 22|. When the subscriber hearsthis tone he hangs up his receiver. At the same time, as explainedabove, tube V is blocked by potential U4 which is applied by conductor41 from point f of circuit Vlt; there is no modulating potential acrossresistance R53 so that the second triode of tube V|8 remains blocked andthe calling subscriber cannot overhear the conversation in which thesubscriber he is calling is taking part.

Point f is connected to the grid of tube V|5b by a resistance R68 and tothe grid of tube Vl'lb by a resistance R69 andit is decoupled by acondenser C25.

When, after he has received the busy tone, the calling subscriber hangsup, the positive impulses 1| disappear from conductors 232 and 52 andare no longer applied to the grid of tube V1. The mean current whichflows when there are impulses in the plate circuit of tube V1 disappearsand the potentials at the terminals of resistances R33 and R36 arebrought down from the cut-oir potential to the normal potential of tubeV|0 which unblocks. Relay 404 through which flows the iplate current ofV|0 operates and closes by its make contact the circuit of theelectro-magnet of the step-by-step switches P3, P2, Pl which go back torest by means of the interrupted potential supplied by generator 21 aspreviously eX- plained.

On the other hand the disappearance of impulses 1I stops the controlcircuit 300 and therefore impulses 85D. Tube V|5a is then againconductive and the flip-flop circuit goes back to its free position.

When the called subscriber is free there are no impulses 19D onconductor 50; therefore no impulses reach the grid of tube V8 bycontacts 4331` of relay 403. The nip-ilop circuit 4|6 remains in itsfree position. When relay 403 opens its contact 4331- and 434 and closesits contact 433i, 435 and 43|, the impulses 84D occurring at instantstn, which characterise the called subscriber, are applied on the onehand to the grid of tube V|1b by contact 435 and on the other hand tothe grid of tube V9. They appear with a positive polarity on the plateof tube V|1b which is connected to the high potential through a loadresistance R65 and, by a coupling condenser C26 and conductor 58, areapplied to the grid of tube V2| (Fig. 6). This grid comprises a leakresistance Rll. On the other hand the cathode of this tube is biassedbeyond its cutoff potential for example by a battery B9 through aresistance R12 and its plate is connected to a load resistance R13. Thepositive impulses from tube V|1b bring this tube V2| to the cutoffpotential. The current of the signalling gen-`A erator 25 flows throughtransformer T4, conductor 48 and closed contact 43| through tube V22which is unblocked. The cathode of tube V22 is bassed for instance by abattery BH); its anode is connected to a load resistance R15 and anappropriate grid potential is applied to it from point f through aresistance R14. The ringing current from generator 25 iiows through thistube and, through a coupling condenser C21, is applied to contact 43|.Resistance R16 constitutes another load. This current is then applied toconductor 40 and to the grid of tube V|8 which is unblocked at thismoment. As long as the called subscriber has not lifted his receiver noimpulse from conductor 232 is applied to the grid of tube V8 byconductor 5| and contact 43315. Since the plate current of this lasttube is Zero the potential drop across resistances R53 and R65 whichbiassed the grid of tube V|8 with respect to its cathode, supplies tothis tube a cut-01T potential. When the called subscriber lifts hisreceiver impulses 'HD appear on the grid of tube V2. At this moment aplate current flows in this tube and causes in resistances R63 and R65 apotential drop which brings the bias voltage of tube V|8 beyond itscut-olf potential.

It may be noted that the opening of contact 43| during the transmissionof digit impulses prevents the sending of the dialling tone to thecalling subscriber.

At the output of tube V|S the potential induced by the ringing generator25 is applied simultaneously to the grid of tube V20 for the callingsubscriber and V2| for the called subscriber by points b and drespectively. As explained previously, since tube V20 is unblocked byimpulses 13D and tube V2| is unblocked by impulses 84D, which have flownthrough tube V|1b, the calling subscriber hears through tube V20 theringing current sent to the called subscriber through tube V2 On theother hand impulses 84D show that subscriber All is busy. These impulsesare applied by contact 435 of relay 403 to conductor 45 and reach thegrid of tube V9. The cathode of this tube is brought to an appropriatebiassing potential by resistance R11 and a load resistance R18 isprovided in its plate circuit. Impulses 84D iiow through tube V3 andappear as positive impulses 86D at instants tn these latter impulses areapplied to the cathode Kl of tube VG through a condenser C5 and byconductors 43 and 244 as explained previously. They bring the potentialof the grid of tube VG beyond its cut-off potential at instants in andthus prevent the impulses tn, produced by the called subscriber -when hehas lifted his receiver, from reaching the test line 242 and seizinganother connecting circuit.

On the other hand Contact 435 is closed only after the opening ofcontacts 434 and 4331 to avoid superfluous changes in the condition ofcircuit 4|6.

When the called subscriber lifts his receiver the impulses HD atinstants tn which are thus generated, can iiow only on line 232, since,it has just been seen, tube VG is `blocked at instants tn. Theseimpulses 1|D ow on conductor 5I, contact 43372 of relay 403 and areapplied by conductor 49 to the grid of tube V8 which is unblocked at thesame instant, as seen previously, by impulses 84D and the D. C.potential U3. These impulses therefore cause a plate connection isrequired is busy, an electronic gate. means for causing said :flip-flopcircuit when in its second position to close said electronic gate at theinstants characterizing the calling channel forv preventing; the callingchannel from being connected to the channel with which connection isrequired.

5. A switching system as claimed in claim 4 in which the rst electronicgate is closed by the action of the impulses from the control circuit ofeach busy connecting circuit, thereby diverting the flow of the impulsesfrom the calling channel through an alternative path to the connectingvcircuit, a second electronic gate in each connecting circuit controlledby impulses from the control circuit and opened only at the instantswhen the impulses controlling the said circuit occur, and applying theimpulses from the calling line to said controlling circuit, a thirdelectronic gate in Veach connecting circuit controlled by other impulsesfrom the control circuit of this connecting circuit, opened between theinstants when the impulses controlling said circuit occur and enablingthe impulses of other calls to reach the following circuits, a fourthelectronic gate in each connecting circuit opened only at the instantswhen the impulses controlling this circuit occur and enabling theimpulses following the first one to reach this circuit by the saidalternative path, and a fifth electronic gate in each connecting circuitcontrolled by the impulses delayed at the instants characterisinganother channel to which connection is required and opened at theseinstants to let pass towards a busy condition circuit the impulses ofthe said other channel when the said other channel is busy.

6. A switching system as claimed in claim 5 having a busy lconditioncircuit comprising a flip-flop circuit controlled by the impulses of abusy called channel transmitted through the said fifth electronic gate,a sixth electronic gate controlled by the said flip-flop circuit `underthe action of the impulses of the calling channel and closed at theinstants characterising this calling channel preventing the said callingchannel from receiving impulses originating in the said busy calledchannel.

7. A switching system as claimed in claim 3 comprising in combination anelectronic distributor Whose electrodes are respectively connected to aplurality of channels of communication, a rst electronic gate connectingthe common electrode of the distributor to the rst of the electronicconnecting circuits, the said electronic gate being locked at theinstants corresponding to a calling channel, a plurality of registercircuits each associated respectively with a connecting circuit, anddelay devices adjusted by the register circuit associated with theconnecting circuit under the control of the calling channel andimparting to the impulses of the calling line a delay corresponding tothe instants characterizing the said called channel.

8. A switching system of the type wherein a number of channels ofcommunications are represented by individual time positions in arecurring cycle of time positions comprising a common transmissionmedium for said channels, a rst and a second connecting circuit,responding means in each of said connecting circuits for responding toan impulse at any time position, a connection from said transmissionmedium to both said responding means, blocking means interposed betweensaid transmission medium and said second connecting circuit normallyblocking the passage thereto of impulses from said transmission medium,means responsive to the reception of an initial impulse by theresponding means of said first connecting circuit for by-passing anysubsequent impulses at other time positions and for unblocking saidblocking means with respect to impulses only of said other timepositions.

9. A switching system of the type wherein a number of channels ofcommunication are represented by individual time positions in arecurring cycle of time positions comprising a common transmissionmedium for said channels, a rst and a second connecting circuit,responding means in each connecting circuit for responding initially toan impulse at any time position, a connection from said transmissionmedium to both said responding means, blocking means interposed betweensaid medium and said second circuit normally blocking the passagethereto of impulses from said medium, means controlled by saidresponding means for by-passing any subsequent impulses at other timepositions to the next succeeding connecting circuit, and for unblockingsaid blocking means only with respect to impulses at time positionsother than that of the impulses causing said initial response.

l0. A switching system wherein a number of channels of communication arerepresented by individual time positions in a recurring cycle of time.positions comprising a common transmission medium connected to saidchannels, means between said medium andsaid channels for feedingimpulses to said medium representative of calling channels, a pluralityof connecting circuits, means in each of said circuits responsive toimpulses at any time position, an input circuit for each of saidconnecting circuits, one of said input circuits being connected to saidcommon medium, means controlled by the responsive means of each circuitwhen an impulse has been received thereby for preventing subsequentrepetitions of the same impulse from being received by the input circuitof any connecting circuit, and means also controlled by said responsivemeans when an impulse has been received thereby for disconnecting saidresponsive means from its associated input circuit during the times ofthe other impulses of said cycle and connecting said input circuit tothe input circuit of another connecting circuit during such times.

`STANISLAS VAN MIERLO. BORIS DERJAVITC'H.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,905,359 Aiel Apr. 25, 1933 2,277,192 Wilson Mar. 24, 1942`2,341,919 Hubbard Feb. 15, 1944 2,356,514 Graham Aug. 22, 19442,356,519 Hubbard Aug. 22, 1944 2,379,715 Hubbard July 3, 1945 2,387,018Hartley Oct. 16, 1945 2,462,896 Ransom Mar. 1, 1949 2,490,833 RansomDec. 13, 1949

